1. Field of the Invention
The present invention relates to a zoom lens barrel which can be advantageously used with a compact camera having a photographing optical system and a separate finder optical system.
2. Description of Related Art
In a recent zoom compact camera, a zoom lens can be retracted beyond a photographing position in which a picture can be taken to reduce the storage length of the zoom lens. In this compact camera, in case where separate tracks can be provided for respective lens groups of the zoom lens, return tracks thereof can be optionally set. However, in case of a zoom lens camera having more than two lens groups of which two or more lens groups move together, there is a limit to the storage length of the zoom lens.
A zoom lens having more than two groups of lenses is known as a so-called vari-focal lens. The vari-focal lens has an advantage that it is easy to achieve a high variable power. However, a vari-focal lens is usually not used in a single-lens reflex camera, since a shift of focus (a change of focus point) occurs during zooming. On the contrary, in an auto-focus compact camera which has a separate photographing optical system and a finder optical system, the shift of focus which takes place during zooming can be compensated for by the displacement of the focusing lens group. Namely, since the focus position of the focusing lens group on which a focused image of an object to be taken can be formed on a film plane can be calculated in advance in accordance with the focal length and the object distance, it is possible to move the focusing lens group to the the calculated position to compensate for the shift of focus. Accordingly, in a compact camera, the vari-focal lens can be used as a zoom lens. Note that although a lens in which no shift of focus takes place during zooming is usually called a zoom lens, and a lens in which the shift of focus takes place during zooming is usually called a vari-focal lens, both lenses will be referred to as a "zoom lens" in the present specification.
In a vari-focal lens having three lens groups, the focusing is effected by a second lens group which is located between a first lens group and a third lens group. Generally speaking, in a zoom lens, upon assembly, the focusing lens group is moved in the optical axis direction to effect a zooming adjustment. The zooming adjustment is an adjustment in which no shift of focus occurs when the zooming is effected at a reference object distance (usually, the infinite distance). However, in a vari-focal lens having three lens groups, it is very difficult to perform the zooming adjustment by the second lens group which is located between the first and third lens groups.